1. Field of the Invention
The present invention relates to an amplifier, more particularly to a class AB amplifier.
2. Description of the Related Art
Referring to FIG. 1, a conventional class AB amplifier comprises a voltage amplifier stage 11 and a power amplifier stage 12. The conventional class AB amplifier amplifies a differential input voltage and converts a resulting amplified voltage into an output current. The voltage amplifier stage 11 typically uses a floating gate structure, and includes first to third p-channel metal-oxide-semiconductor field-effect transistors (PMOS transistors) 111-113, and first to third n-channel metal-oxide-semiconductor field-effect transistors (NMOS transistors) 114-116. Alternating current fluctuations of a voltage at node (A) are related to the differential input voltage. The power amplifier stage 12 includes a fourth PMOS transistor 121 and a fourth NMOS transistor 122.
The voltage amplifier stage 11 and the power amplifier stage 12 must operate using the same source voltage (VDD) due to the fact that the voltage amplifier stage 11 is able to operate normally only when the following equations are satisfied:Vgs,114+Vgs,122=Vgs,115+Vgs,116′Vsg,111+Vsg,121=Vsg,112+Vsg,113′
where Vgs,X is a voltage difference between a gate and a source of NMOS transistor (X), and Vsg,X is a voltage difference between a source and a gate of PMOS transistor (X).
However, a drawback of the conventional class AB amplifier described above is that a high voltage swing and a high current drive capability of the power amplifier stage 12, which are obtained by using a high source voltage (VDD), are achieved at the expense of a high power consumption of the voltage amplifier stage 11.